ACI Certification Concrete Mix Design 3

Question 1

In a concrete mix, 'paste volume' refers to the combined volume of:

Accepted Answer

Cement, water, and entrained air

Answer

Paste is the binding matrix composed of cement, water, and entrained air that coats and bonds aggregate particles together.

Question 2

When field test records show a standard deviation of 400 psi for concrete with f'c = 4,000 psi, the required average strength (f'cr) using f'cr = f'c + 1.34s is:

Accepted Answer

4,536 psi

Answer

f'cr = 4,000 + 1.34 × 400 = 4,000 + 536 = 4,536 psi; this formula applies when adequate field test records exist.

Question 3

What is the primary benefit of air entrainment in concrete exposed to freezing and thawing cycles?

Accepted Answer

It creates small voids that relieve hydraulic pressure from freezing pore water

Answer

Entrained air bubbles act as pressure-relief chambers, allowing expanding ice to move into voids rather than fracturing the cement paste.

Question 4

How does increasing the maximum aggregate size generally affect the water requirement for a given slump?

Accepted Answer

Water requirement decreases

Answer

Larger aggregates have less total surface area per unit volume, reducing the water needed to coat particle surfaces and achieve the target workability.

Question 5

Moisture corrections to batch weights are required whenever aggregate moisture content is:

Accepted Answer

Above or below the SSD condition

Answer

Aggregate above SSD contributes free water to the mix while aggregate below SSD absorbs water from it—both conditions require correcting the batch water quantity.

ACI (American Concrete Institute Certification) Practice Test

ACI (American Concrete Institute Certification) Practice Test

ACI Certification Concrete Mix Design 3